Many eyeglass lenses and other optical products are now being made of synthetic resins because of cost, weight, strength and processing considerations. When they can be molded with suitably precise curvatures and high purity, optical plastics can be employed wherever temperature sensitivity, index of refraction, and dispersion characteristics are not too limiting. The most used resins for these purposes are polycarbonates, which can be molded or cast by production equipment into finished or semi-finished form at relatively lower cost than glass, despite the much longer history of glass technology. Plastics have excellent impact resistance, and although they are less abrasion resistant than glass, hard thin coatings are now available that can overcome this deficiency.
Special requirements must be met in casting eyeglass lenses from synthetic resins. The requirements are quite severe and have resulted in domination of the plastic eyeglass market by one particular formulation. The cast and finished product must not only be clear and without noticeable coloration, but must be free of impurities, bubbles and striations. The materials should not attack or be incompatible with the molds and gaskets used in the casting process. They should have relatively low sensitivity to process conditions, with the consequent low yields that such criticality usually causes.
In most instances, the eyeglass blank is cast in thick semi-finished form, with its convex (outer) surface having a specified curvature. The inner (concave) side is then ground and polished down to the desired final lens thickness and the needed curvature for a given optical prescription. Consequently, the material must abrade in such manner that it does not adhere to or clog the finishing surface or abrasive material and slow the finishing operation. Currently, five steps are used in forming and finishing a contour and if this procedure can be shortened, substantial benefits can be realized by optical finishing laboratories.
In addition, hardness, strength, and impact resistance must meet certain minimum levels. For example, the material must be hard and tough enough to withstand, without cracking, the dropping of a standard steel ball from a certain minimum height. Resistance to notch sensitivity should be high so that there is a reasonable margin of safety in the event of impact.
One drawback in the manufacture of plastic ophthalmic lenses relates to the index of refraction of the underlying lens material. Plastic materials tend to have a low density and relatively low indices of refraction. The index of refraction of plastic lenses should approximate or exceed that of the glasses most widely used. If the index of refraction is relatively low, e.g. less than 1.5, thick lenses are required for high corrections. Materials having higher indices than about 1.55 are currently sought, because the lens can then be thinner and lighter for a given correction. In reducing lens thickness, however, improved mechanical properties are usually needed in order to maintain appropriate margins of safety.
The material must also, for eyeglass applications, be capable of compatibly receiving, and firmly bonding to, the available abrasion-resistant coatings, a number of which are now in use. Preferably, it should also be capable of receiving a tintable coating in any of a wide range of colors.
Generally, plastic lenses have been made from a variety of conventional plastic materials, such as polycarbonate, polyethylmethacrylate, and polyallyldiglycol carbonate. For many decades, the principal optical plastic used for making eyeglasses has been "CR-39", a polycarbonate product of PPG Industries. It has met in adequate degree all of the significant requirements as to optical properties, strength, index of refraction, cure time, processing criticality and compatibility with coating and tinting materials. Although improved over the years, its cost is relatively high, the index of refraction is only in a medium range, and its processing properties require relatively long cure times and involve substantial shrinkage.
Acrylics and polyesters have been given consideration over the years because they are inherently lower cost materials than the polycarbonates. As examples, U.S. Pat. Nos. 3,265,763, 3,391,224, and 3,513,224 record prior efforts to cast eyeglasses using polyester resins. U.S. Pat. No. 3,391,224, for example, proposes the use of a major proportion of an esterification reaction product, together with from 5% to about 20% by weight of methyl methacrylate and less than suitable for ophthalmic lenses. The esterified reaction product is mixed with the methyl methacrylate, thoroughly blended, and then catalyzed with 1.5 parts of benzoyl peroxide. Following curing for in excess of 17 hours in a casting cell at an elevated temperature, a grindable optical lens is said to be produced, but no figures are given for hardness, impact resistance, or index of refraction.
An attempt to achieve a balance of properties suitable for eyeglass lenses using polyesters and acrylics is evidenced by U.S. Pat. No. 3,513,224 to Sherr. The major portion of the composition taught by that patent is the esterification reaction product of fumaric acid, trimethylene glycol and neopentyl glycol, together with 12% to 18% of styrene and 8% to 12% of ethylene glycol dimethacrylate, suitably catalyzed after mixing. The styrene was used to increase the refractive index, but had a tendency to induce extensive cross-linking and therefore brittleness which was counteracted by the ethylene glycol dimethacrylate, which independently aided the impact strength. While this composition was said to be suitable for casting lenses, it required heating at 60.degree. C. for 16 hours, then 100.degree. C. for 90 minutes and finally gradual heating to 135.degree. C. The resultant lens had a Barcol hardness of only 31 and a refractive index of 1.5248, which properties do not meet current norms. Furthermore, the heating times and levels required were much too great.
Some commercial efforts, more recent than the patents listed above, have been reported pertaining to polyester casting materials for eyeglass lenses. These were apparently unsuccessful because of insufficient impact strength. Changes in the formulation were then understood to have been undertaken to improve toughness by increasing flexibility, but it is believed that the resultant material became extremely difficult to contour and polish. The products now appear to have been withdrawn from the market. A polyester based optical compound of about 1.56 index of refraction is now being offered on a commercial basis, however.
The current state of the art as to high index materials is summarized by an article by K. Angel in the magazine Optical Index for May 2, 1988 entitled "The High Index Race is on to Introduce Superior Products" (p. 24). This describes how the thinner, lighter lenses are expected to be increasingly employed for progressive, multifocal and higher correction lenses.
The relative cost of "CR-39" is only one reason that improved optical plastics are being sought for eyeglasses. "CR-39" undergoes substantial shrinkage when curing, so that most lens casting systems which use this material are adapted to compensate for shrinkage in such a way that they can form only one or two lenses at a time. The length of the cure cycle also increases processing and capital equipment costs. For long term storage, "CR-39" and its catalytic agents must be held at very low temperatures, because at ambient temperatures they tend to become explosive with time. The basic polycarbonate system is also quite reactive, and requires an alkaline catalyst, so that it gradually but persistently attacks molds, retainers, and other associated elements. This means that the molds have a limited life, and must be replaced regularly, further increasing costs.
In the casting of plastic eyeglasses, the resin is introduced between spaced apart glass or metal mold surfaces, encompassed about their periphery by a resilient gasket or retainer ring that is non-reactive, or substantially non-reactive, to the polymers being cast. It must therefore flow and remain stable for a reasonable interval without entraining bubbles or creating voids. As the material cures, generally under elevated temperature, pressure is applied to one or both of the dies to compensate for shrinkage, with the ring gasket compressing to accommodate the dimensional change. Examples in the patent literature are numerous, including U.S. Pat. Nos. 3,316,000, 3,240,850, 3,605,195, 3,211,811, 3,240,854, 3,422,168, 3,070,846, 3,056,166, 3,109,696, 4,191,717, 4,227,950, 4,197,266, 4,251,474, 4,257,988, 4,279,401, 4,273,809 and 4,284,591.
Most commercial eyeglass lens casting today, using "CR-39", is based upon the use of individual molds because of the mentioned shrinkage and control problems. However, many efforts have been made to cast lenses simultaneously in multiple molds, as evidenced by U.S. Pat. Nos. 3,871,610, 3,,806,079, 3,871,803 and 3,423,488. It is not known that these approaches have resulted in any successful high yield production manufacture, because commercial processes still largely rely on single or dual mold machines. Nonetheless, the potential benefits of using multiple mold casting machines are evident, if adequate yields of high quality products can be attained.
Standard corrections can be used for a substantial fraction of the eyeglass wearing population. About 50% of single lens corrections are accounted for by a limited number of curvatures. For these users, substantial savings can be realized if the lenses can be cast directly to prescription, as long as quality standards can be maintained. However, temperature, pressure and resin variations must be very closely controlled and the problems of casting to prescription curvatures are excessive with currently available materials.
The storage and processing characteristics of a resin system for casting optical products can be regarded as having almost equal importance to finished product properties. The fact that many chemical alternatives are available in a resin system to alter viscosity, strength, or toughness does not mean that all final objectives can be achieved. Styrene is incorporated in significant amounts in polyester resin systems to make the mass flowable and because it cross links on polymerization. When the styrene is increased, there is no marked increase in index of refraction. However, increased styrene has a tendency to increase brittleness and accelerate exothermic reactions resulting in fractures and discoloration within the coating. Improving index of refraction while also obtaining high impact strength, high resistance to notch sensitivity, low cure time, excellent casting properties and enhanced finishing characteristics therefore requires overcoming significant problems.
Optical plastics having superior properties for single vision eyeglasses obviously can be used with benefit in multi-vision glasses (bifocals and trifocals) as well. Moreover, they can be used for other optical applications, such as single or multi-element light focussing lens systems. The capability of plastics for complex curvatures such as aspherics and fly's eye lenses is well recognized. In addition, the optical properties of high index materials create potential for use in other applications, such as prisms, multi-faceted bodies and fresnel lenses.